This invention relates generally to the field of magnetic data storage devices, and more particularly, but not by way of limitation, to abnormal magnetoresistive element detection for a disc drive.
Disc drives are used for data storage in modern electronic products ranging from digital cameras to computers and network systems. Typically a disc drive includes a mechanical portion and an electronics portion in the form of a printed circuit board assembly that controls functions of the mechanical portion while providing a communication interface to a host being serviced by the disc drive.
Typically, the mechanical portion, or head-disc assembly, has a disc with a recording surface rotated at a constant speed by a spindle motor assembly and an actuator assembly positionably controlled by a closed loop servo system for use in accessing the stored data. The actuator assembly commonly supports a magnetoresistive read/write head that writes data to and reads data from the recording surface. Normally, the magnetoresistive read/write head uses an inductive element, or writer, to write data to and a magnetoresistive element, or reader, to read data from the recording surface.
The disc drive market continues to place pressure on the industry for disc drives with increased capacities, higher data rates and lower costs. A key aspect of achieving lower costs is an identification of marginal components as early as practical in the manufacturing process to preclude needless accrual of additional manufacturing costs and costly rework operations in subsequent processes.
A critical component of a disc drive is the magnetoresistive read/write head. As each read/write head passes through manufacturing processes in preparation for use in a disc drive, costs associated with those processes accrue and contribute to the overall cost of the disc drive. By measuring characteristics of the read/write head throughout the manufacturing process, defective and marginal read/write heads can be culled from the process before additional costs are needlessly applied.
One such characteristic of concern is the resistance of the magnetoresistive element relative to a range of acceptable resistance values. High or low magnetoresistive element resistance values, which are not within the range, are considered defective. High resistance can indicate a discontinuity in the magnetoresistive element, caused by electrical overstress or electrostatic discharge. Low resistance could cause high current surge into the transducer, and lead to electrical overstress.
Read/write head testing methodologies such as RHBUFF (Read Head Buffered) and DBHV (Digital Buffer Head Voltage) method have been employed to cull substandard read/write heads from the process. However, in application each methodology has shortcomings.
The RHBUFF methodology requires extensive modifications to the pre-amplifier and special bias circuits, which lower the noise contribution from the bias circuits to the test results. During testing, a substantially noise free bias current is applied to the MR element and a mid-range frequency response of the MR element is monitored. Correlation between the test results of the RHBUFF methodology and actual performance of the read/write head in a disc drive environment is tentative.
The DBHV method utilizes a comparator, typically incorporated within the application specific integrated circuit ASIC of the disc drive, for comparing a measured voltage across the magnetoresistive element to a pre-set trip voltage. Do to a quantization error inherent in the measurement method, a problem of inaccuracy arises with use of the DBHV method. Typically, the 7-bit setting in the voltage allows the stepping of roughly 6 mV, causing the quantization loss.
As such, challenges remain and a need persists for effective techniques for identifying, testing and predicting operating characteristics of read/write heads throughout the disc drive manufacturing process. It is to this and other features and advantages set forth herein that embodiments of the present invention are directed.
As exemplified by preferred embodiments, the present invention provides a magnetoresistive element measurement circuit configured for measuring resistance of a magnetoresistive element biased by a current source is disclosed. In a preferred embodiment, the magnetoresistive element measurement circuit includes the magnetoresistive element. A current source provides a fixed current that biases the magnetoresistive element. Upon activation of at least one current bypass switch within the circuit, a measurement resistor communicates with the biased magnetoresistive element in an electrical parallel configuration. The fixed current splits and flows through the measurement resistor while continuing to bias the magnetoresistive element. Included in the measurement circuit is an analogue voltage detector that first measures the voltage across the biased magnetoresistive element with the measurement resistor switched out of the circuit and then measures the voltage of the circuit with the measurement circuit switched in the circuit. Each of the measured voltages in combination with the measurement resistor are factored together to determine the resistance of the magnetoresistive element. The determined resistance of the magnetoresistive element is compared to a resistance range, if the determined resistance is within the range the magnetoresistive element is in an operative state. However, if the determined resistance is outside the range the magnetoresistive element is in a non-operative state. These and various other features and advantages, which characterize the present invention, will be apparent from a reading of the following detailed description and a review of the associated drawings.